A photo-coupled biomeasuring system using a liquid crystal optical modulator has been studied experimentally. Using this system, consisting of an optical transmitter, a receiver and two optical fibers, patients are fully isolated from ac lines. The system helps to make patient monitoring safe as electric shock hazard have been eliminated. Excessive noise induction along the metallic leads can also be avoided. Practical use of photo-coupled biomeasuring system primarily depends upon the power consumption of the optical transmitter energized by dry batteries. A liquid crystal optical modulator is introduced as the driving power of optical modulator is fairly lower than driving LED's. The optical modulator made of the conventional liquid crystal display panel material usually exhibits very slow responses. However, if it is driven by AM signals, the optical output becomes linear to the envelope of the AM signals, and its frequency response extends up to a fairly high level of eq. 500 Hz, with-6dB/oct drop. The amplitudes of optical signals are distorted by motion of the fibers. In optical FM signals, this distortion is avoided, because the amplitudes of FM signals have no information. Then, the liquid crystal optical modulator is driven by FM-AM technique to make optical output signals into FM waveforms. It can be realized that an overall frequency response of the system is fiat between 0.1 Hz and 100 Hz with the power consumption of 1 mW in the optical transmitter. The characteristics of the system is verified by recording ECG's generated by an ECG generator.